1. Field of the Invention
This invention relates to a control system for vehicle having a continuously variable transmission.
2. Description of the Related Art
Conventionally, a control system of the above-mentioned kind has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H4-285361 and Japanese Laid-Open Patent Publication (Kokai) No. H9-112674. The continuously variable transmission is comprised of a drive pulley connected to an engine, a driven pulley connected to drive wheels, a transmission belt for transferring a driving force between the two pulleys, and oil pressure control valves. The drive pulley has a moving part and a fixed part. The moving part is fitted on a pulley shaft such that the moving part is axially movable along the shaft but inhibited from rotation thereabout. The fixed part is fixed to the pulley shaft and opposed to the moving part. The moving part, the fixed part, and the pulley shaft define a V-shaped groove. The driven pulley has the same construction as that of the driven pulley. The transmission belt is stretched around the drive pulley and the driven pulley such that it extends in the grooves thereof. Connected to the moving parts of the drive pulley and the driven pulley via the oil pressure control valves is an oil pressure pump using the engine as a drive source. The oil pressure control valve is controlled depending on the operating conditions of the engine, whereby the oil pressure supplied to at least one of the drive pulley and the driven pulley from the oil pressure pump is controlled. This causes the moving parts to move to change the respective effective diameters of the grooves, whereby the transmission ratio is continuously controlled and at the same time the transmission belt is sandwiched between the moving part and the fixed part of each pulley, for prevention of slippage.
The control system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H4-285361 is configured such that when the vehicle is traveling on a bad road, the sandwiching force applied to the transmission belt is increased. This is with a view to the following purpose: Assuming that the vehicle is traveling on a bad road with a very rough road surface, the drive wheels rotate without load when they move over a bump, resulting in a sudden increase in the rotational speed of the engine, and then, the drive wheels land on the road surface to receive a very large resistance to the rotation of the drive wheels, developing inertia torque. The inertia torque makes the drive pulley and the driven pulley unbalanced in rotation, imposing increased load on the transmission belt, so that there is a fear of slippage of the transmission belt. The sandwiching force is increased for prevention of the slippage.
However, in this conventional control system, when the vehicle is traveling on a bad road, the sandwiching force of the transmission belt is increased with load being placed on the transmission belt, which further increases the load on the transmission belt, resulting in reduced service life thereof. Further, when the vehicle is traveling on a bad road, the inertia torque generated upon landing of the drive wheels on the road surface is transmitted to the internal combustion engine of the vehicle, causing a change in the rotational speed thereof, which degrades drivability. Further, to increase the sandwiching force applied to the transmission belt, it is necessary to increase the output of the oil pressure pump, which increases load on the engine to lower fuel economy. Further, on a bad road, the vehicle lurches violently, so that the accelerator pedal is frequently stepped-on and released at a very short time period in spite of the driver's intention. As a consequence, in the case where the degree of opening of the throttle valve is controlled according to the stepped-on amount on the accelerator pedal and the transmission ratio of the continuously variable transmission is controlled according to the degree of opening of the throttle valve, when the vehicle is traveling on a bad road, due to the control of the degree of opening of the throttle valve responsive to the stepped-on operation of the accelerator pedal, the degree of opening of the throttle valve is fluctuated, causing fluctuation of the transmission ratio controlled according thereto. As a result, the rotational speed of the engine fluctuates, also causing degraded drivability.
Further, in the control system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H9-112674, assuming that the transmission ratio of the continuously variable transmission is decreasing, the sandwiching force applied to the transmission belt is increased if a target inertia torque calculated e.g. based on the ratio between the rotational speed of the engine and that of the driven pulley is large, and at the same time, the vehicle speed is low. Under such a condition, the torque of the driven pulley largely increases, placing increased load on the transmission belt, so that there is a fear of slippage of the transmission belt. The sandwiching force is increased for prevention of the slippage.
However, according to this conventional control system, since the sandwiching force applied to the transmission belt is increased in a state where the large load is placed on the transmission belt, further increased load is placed on the transmission belt, which reduces the service life thereof. To increase the sandwiching force applied to the transmission belt, it is required to increase the output of the oil pressure pump. Further, since the target inertial torque is calculated e.g. based on the ratio of the rotational speed of the engine and that of the driven pulley, and the transmission belt is sandwiched by the control of oil pressure executed according to the result of the calculation, there is an inevitable delay in the control. To cope with this inconvenience, it is necessary to constantly set the pressure of the oil pressure pump to a little too high a level, which degrades fuel economy of the engine that drives the oil pressure pump, and further reduces the service life of the transmission belt. To solve this problem, it is contemplated to employ a continuously variable transmission having a large torque transfer capacity. However, this brings about increases in the size and manufacturing costs of the transmission. Further, it is also a conventionally known method to allow slippage of the clutch to suppress a jerky feeling occurring when the accelerator pedal is suddenly stepped on or released. However, in this case, to cope with inertia torque caused by lowered rotational speed of the engine when the clutch is engaged again after slippage thereof, it is also required to set the pressure created by the oil pressure pump to a little too high a level. This brings about the same problem described above.